The present invention relates to coated cemented carbide cutting tools with improved properties obtained by a heat treatment of the cemented carbide before the application of a wear resistant coating. The invention is, in particular, applicable to WC+Co based cemented carbide but can also be applied to cemented carbide of WC+Co+gamma phase (gamma phase is a common name for solid solution carbide, mainly comprising W as well as Ti, Ta and Nb).
A common method for achieving an improvement in toughness of coated cemented carbide cutting tool inserts is by various types of gradient sintering methods in which Co enriched surface zones are formed. Two major methods are used.
In one method, an addition of nitrogen in the form of TiN or Ti(C,N) to WC—Co-gamma phase grades is used, which during sintering develops a Co-enriched surface zone free from gamma phase with a thickness of up to 30 μm.
In the other method, a controlled slow rate of cooling down from the sintering temperature is used, whereby a Co-enriched surface zone having Co in the form of a stratified structure is formed. This is achieved in WC—Co-gamma phase or WC—Co based cemented carbide having a carbon content over the carbon saturation point and thus containing free graphite.
U.S. Pat. No. 4,830,930 discloses a surface refined sintered alloy body which comprises a hard phase and a binder phase. The concentration of the binder phase in the surface layer is highest at the outermost surface thereof and approaches the concentration of the inner portion, the concentration of the binder phase decreasing from the outermost surface to a point at least 5 μm from the surface. The method for making the same includes applying a decarburization treatment at the surface of the sintered alloy at temperatures within the solid-liquid co-existing region of the binder phase after sintering or in the process of sintering.
U.S. Pat. No. 4,830,886 discloses a process for forming a coated cemented carbide cutting insert by chemically vapor depositing a layer of titanium carbide under suitable conditions to form a titanium carbide coated insert with eta phase in the cemented carbide substrate adjacent to said titanium carbide coating. Subsequently, the titanium carbide surface is contacted with a carburizing gas for a sufficient time and at a sufficient temperature to convert substantially all of said eta phase to elemental cobalt and tungsten carbide. U.S. Pat. No. 5,665,431 is similar but relates to a titanium carbonitride coating.
WO 99/31292 discloses a body of cemented carbide provided with at least one wear resistant layer, which body contains a zone in the cemented carbide and, adjacent to the applied layer, containing triangular WC platelets with a specific orientation.
WO 98/35071 relates to a method comprising the steps of: a) removing carbon from a surface layer of a cemented carbide substrate at a temperature in the region of about 900° C. to about 1400° C. and in an oxygen-containing atmosphere b) reintroducing carbon into the surface layer of the substrate at a substrate temperature in the region of about 900° C. to about 1400° C. in a carbon-containing atmosphere; and c) coating the substrate with a hard material.
WO 00/31314 describes coated tools and a method of manufacture. The process includes formation of an eta phase-containing surface zone, and a conversion treatment in at least a partial vacuum during which a surface is obtained with microroughness greater than 12 microinches and comprising eta phase and fibrous tungsten carbide grains.
EP-A-0 560 212 describes coated cemented carbide with a Co-enriched surface zone used for cutting tools and having improved resistance to chipping without sacrificing wear resistance. Zr and Hf comprising phases are present within the cemented carbide. The Co enriched surface zone comprises WC grains with increased grain size compared to the inner parts of the cemented carbide.